Dry impact capture of aerosol particulates

ABSTRACT

A dry method and apparatus for treating an effluent gas stream in order to facilitate the removal therefrom by conventional means of contaminating particulates, particularly those in the submicron range. Target particulates which are larger than the submicron contaminating particulates are dispersed in a secondary gas stream. The secondary gas stream is then introduced into the effluent gas stream. The manner of introduction is such that the contaminating particulates impact with and are captured on the larger target particulates. The target particulates and the contaminating particulates inertially impacted thereon are then separated from the combined effluent and secondary gas streams by conventional gas cleaning equipment.

This is a continuation of application Ser. No. 892,881 filed Apr. 5,1978, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a dry method and apparatus for treating aneffluent gas stream in order to facilitate the removal therefrom byconventional means of contaminating particulates particularly those inthe submicron range.

2. Description of the Prior Art

In many countries throughout the world, emission standards have been orare being established to control the particulate content of effluentgases being exhausted to the atmosphere. Although these standards varywidely, most limit the particulate content of effluent gases to levelsbelow 0.1 gr./sdcf. In many industrial applications, such as for examplefiberglass furnaces, municipal incinerators, etc., such rigid standardscan only be met by capturing and separating a large percentage of thesubmicron particulates suspended in the effluent gas stream.

Conventional dry cleaning devices lack the ability to effectively andreliably separate submicron contaminating particulate suspended ineffluent gas steams. For example, in the case of baghouses where fabricfilter bags are employed, experience has indicated that the submicronparticulates have a tendendy to rapidly plug or "mask" the fabricinterstices, thus requiring frequent disruptive bag shaking operations.Where cyclone dusts separators are employed, the submicron particulateshave been found to lack the necessary mass required for efficientcentrifugal separation.

Wet venturi scrubbers have also been employed for the purpose ofseparating contaminating particulates from effluent gas streams.Basically, a wet venturi scrubber consists of a constriction in theconduit carrying the contaminated effluent gas stream. The effluent gasstream is accelerated through the venturi constriction, and a liquid(usually water) is injected into the gas stream at the venturi throat.The high gas velocity atmoizes the liquid and the relative velocitiesbetween the contaminating particulates and the liquid droplets result ina combination of one with the other through inertial impaction. Theliquid droplet and their captured contaminating particulates are thenseparated from the effluent gas. While this technique can lead to highercollection efficiencies, this advantage is offset to a considerableextent by other associated problems.

For example, it is known that the efficiency of the inertial impactiontechnique can be improved by reducing the size of the target liquiddroplets. This however requires higher gas velocities with accompanyingpressure drops across the venturi of 30"-60" w.g. A pressure drop of 30"w.g. results in an excessively high energy usage of 240 kWh per millioncu. ft. of gas cleaned. Attempts at reducing the pressure drop acrossthe venturi have not been successful, primarily because a high gasvelocity is essential at the venturi throat in order to achieve optimumatomization of the injected liquid and still have a remainingdifferential velocity between the liquid droplets and the contaminatingparticulates which is sufficient to produce the desired inertialimpaction. Some thought has geen given to injecting a pre-atomizedliquid spray into the gas stream in order to accommodate reduced gasvelocities through and reduced pressure drops across the venturi, butany advantage gained in this regard has been found to be offset by thepower required to pre-atomize the liquid.

Another problem with wet venturi scrubbers is that the atomized liquiddroplets combine with acid components of the effluent gas stream toproduce a high corrosive medium. This in turn makes it necessary toemploy ducts and associated downstream equipment constructed ofexpensive exotic corrosion resistant materials. Even when this is done,however, corrosion related maintenance problems are encountered.Moreover, the resulting acid solutions must be neutralized, and evenafter this is done, disposal problems are encountered.

Other known gas cleaning arrangements have involved the injection ofsolid material into the effluent gas stream. An example of one sucharrangement is shown in U.S. Pat. No. 2,875,844. Such arrangements haveresulted in little or no capture of submicron particulates because thesolids have been dumped into the effluent gas stream at the outsidediameter of the conveying duct as a dense agglomerate. By the timedispersion occurs, a condition which is essential for efficient capture,the solids have attained approximately the same velocity as that of theeffluent gas and the contaminating particulates suspended therein.Without an adequate relative velocity between the contaminatingparticulates and the dispersed target particulates, effectiveparticulate capture through inertion impaction is an impossibility.

According to U.S. Pat. Nos. 3,969,482 and 3,995,005, solid particulatematerial for sorbing and/or reacting with acid gases may be dispersed ina secondary air stream and radially injected through one or more pointsalong an effluent gas carrying conduit. However, in the absence ofadequate relative velocity between the gas streams together with meansfor initially distributing the secondary stream into the effluentstream, little or no capture of contaminating particulates occurs,especially at conduit positions radially removed from the injectionpoints. By the time the added material is dispersed throughout theeffluent gas, the relative velocity of the streams approaches zero.

While electrostatic precipitators have met with some success, theiroperation has been plagued by corrosion, buildups of oils and fats wherecombustion practices are less than optimum, and variations in theconductivity of the contaminating particulates.

SUMMARY OF THE INVENTION

It is a general objective of the present invention to obviate or atleast considerably reduce the aforementioned problems and disadvantages.

According to one aspect of the present invention, there is provided adry method for treating an effluent gas stream in order to facilitatethe removal therefrom of contaminating particulates. Target particulatesare dispersed in a secondary gas stream. The secondary gas stream isthen introduced into the effluent gas stream with the relativevelocities of the effluent and secondary gas streams at introductionbeing such that contaminating particulates impact with and are capturedon target particulates. To obtain distribution of the targetparticulates throughout the effluent gas while maintaining relativevelocity, the secondary stream is introduced into the interior of theeffluent stream, or from a plurality of positions about the longitudinalaxis of the effluent stream, or preferably both. Preferably the secondgas stream is introduced into the effluent gas stream countercurrentlythereto, although it is also contemplated to effect the introduction ofthe secondary gas stream either transversely to or cocurrently with theeffluent gas stream.

The target particulates are large enough so that after they combine withcontaminating particulates, the resulting particle size and mass will besuch as to permit efficient capture and separation by conventional gascleaning equipment.

According to another aspect of the present invention, there is providedan apparatus for treating an effluent gas stream comprising a firstconduit for carrying the effluent gas stream along a first flow path,and a second conduit communicating with the first conduit. Means areprovided for producing a secondary gas stream in the second conduit andfor injecting target particulates into the secondary gas stream at alocation sufficiently removed from the outlet end of the second conduitto permit the target particulates to become dispersed by and suspendedin the secondary gas stream before introduction into the effluent gasstream. The exit end of the second conduit communicates with theinterior of the first conduit, or with distributor means for introducingthe secondary gas stream at a plurality of positions about thelongitudinal axis of the first conduit. Distributor means located withinthe first conduit is preferred. The distributor means is preferablyarranged to introduce the secondary gas stream countercurrently into theeffluent gas stream, although alternate embodiments are contemplatedwhere the aforesaid introduction occurs transversely to or cocurrentlywith the effluent gas stream. It is further preferred to employ meansfor accelerating the effluent gas stream during introduction of thesecondary gas stream, which accelerating means may comprise deflectormeans directing the effluent stream toward and away from the firstconduit walls.

The invention will be more clearly understood from the followingdescription and the accompanying drawings, both of which refer topreferred but nonlimiting embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an industrial installationembodying the method and apparatus of the present invention;

FIG. 2 is an enlarged side view, partially broken away, of the preferredembodiment of the injection apparatus of the present invention;

FIG. 3 is an end view of the apparatus shown in FIG. 2;

FIG. 4 is a partially exploded view of the distributor means of theembodiment shown in FIGS. 2 and 3;

FIG. 5 is a side view, with portions broken away, of an alternateembodiment of the invention;

FIG. 6 is a sectional view taken along the line 6--6 of FIG. 5 androtated 90°;

FIG. 7 is a side view, again with portions broken away, of a thirdembodiment of the invention;

FIG. 8 is an end view of the embodiment of FIG. 7; and,

FIG. 9 is a sectional view of still another embodiment of the presentinvention.

Referring initially to FIG. 1, there is shown an optional quench chamber10 receiving an industrial effluent gas S₁ from a conventional source(not shown) such as a municipal incinerator, glass furnace, or the like.The effluent gas S₂ from chamber 10 is directed through first conduit 12to conventional gas cleaning apparatus depicted at 14, which maycomprise a fabric filter such as a baghouse, a cyclone separator, or thelike. At section 22 of conduit 12, a secondary gas stream A₂ isintroduced through second conduit 18 and the resulting gas mixture S₃fed to cleaning apparatus 14 and exhausted to the atmosphere as a gasstream S₄ through stack 16 which may include a conventional exhaustblower (not shown)

The effluent gas S₁ has solid particulates suspended therein, forexample soot, salts, oxides or the like, a portion of which aretypically of submicron size. Such gas also frequently contains acidgases such as CO₂, SO_(x), HCl, HF, and the like. Quench chamber 10 maybe used to cool the gas by aqueous spray, or to remove a portion of theacid gases as salts by alkaline aqueous sprays as described, forexample, in the two above cited patents, the disclosures of which areherein incorporated by reference. The spray water is evaporated andraises the dew point of the gas which may be of assistance in theparticulate capture hereinafter described. It is not necessary in allcases, however, and may be omitted. The spray additions, if used, shouldbe limited to avoid raising the dew point sufficiently high to providesaturation in any part of the process, particularly in apparatus 14where excess moisture can interfere with separation. Preferably adifference of at least about 40° F. or more between dry and wet bulbtemperature is maintained.

The secondary gas stream A₂ is produced in conduit 18 by means of blower20 and introduced into the primary effluent gas stream S₂ at portion 22of first conduit 12, between flanges 24 and 26. Target particulates Pare fed through vibratory hopper 28 and feed screw 30 and injected intostream A₂ at a point within conduit 18 remote from conduit 12 to assuredispersal prior to introduction into the effluent stream S₂. Targetparticulates P can comprise any suitable solid and should have anaverage particle size of at least about 3 microns, preferably 3-50microns, more preferably 3-20 microns, and most preferably 10-20microns. Particulate nepheline syenite or phonolite is preferred.

FIGS. 2-9 and the following description illustrate methods and apparatusfor introducing secondary gas stream A₂ into primary stream S₂ with arelative velocity and distribution sufficient to capture smallercontaminating solid aerosol particulates by inertial impact with targetparticulates P. The mechanism of the inelastic impact capture is notthoroughly understood but is believed to include mass attraction andmechanical effects between the roughened surfaces of the impactingsolids. In some cases, electrostatic forces and humidity effects may beinvolved. Capture has been demonstrated by air elutriation studies inwhich the finer captured particulates were not separated.

The method and apparatus currently preferred are shown in FIGS. 2-4. Acurved upper section 34 of second conduit 18 extends through the wall ofsection 22 of first conduit 12 and has a flange 32 for joining to thevertical portion of conduit 18 shown in FIG. 1. The section 22 issupported by a leg 36. The inner end of section 34 is fixed by supports42a, 42b and 42c and is joined to a first right truncated cone 38 ofsheet material with its larger base directed upstream with respect tothe flow path of gas S₂. Located concentrically within and spaced fromcone 38 is a second right cone 50 having its apex directed downstreamwith respect to the flow of S₂ and upstream with respect to the flow ofgas A₂. Cones 38 and 50 define therebetween an annular passageway forthe secondary gas stream A₂ which terminates in an annular orifice 40through which the stream A₂ is introduced and injected countercurrentlyinto effluent gas S₂. Together they constitute distributor means forintroducing and injecting stream A₂ into stream S₂ at a plurality ofpositions about the axis of duct 12.

A third right cone 52 is provided with its base joined to the base ofcone 50 and with its apex directed upstream with respect to the flow ofgas S₂. Cone 52 serves both as accelerating means for the accelerationof gas stream S₂ during introduction of stream A₂, and as deflectormeans for deflecting the flow of S₂ toward the walls of conduit 12transversely across the orifice 40.

Cone 50 is secured to cone 38 by means of bolts 54 and is spacedtherefrom by means of four spacer members 58. The bolts 54 engage anannular plate 56 secured to the cone 38. As shown in FIG. 3, a hingeddoor 46 is provided upstream of flange 24 for providing access to theinterior of conduit 12. Also shown is a pipe stub 44 for connection to afume hood (not shown) over hopper 28 to prevent escape of targetparticulate to the atmosphere.

By the method and apparatus shown in FIGS. 2-4, the gas stream A₂ anddispersed target particulates are introduced into the interior of streamS₂, with good distribution and high relative velocity. Relative velocityas used herein refers to the algebraic sum of vectors of the flow of gasstream S₂ and A₂ parallel to the axis of duct 12 during introduction ofA₂ into S₂. A relative velocity of about 20-200 feet per second ispreferred and from about 50-150 is more preferred. By the means shown,an efficiency of capture of submicron particulates equivalent to aliquid venturi having an inlet to throat pressure drop of from 50 to 150inches water gauge can be obtained with substantially reduced power,without contaminated wash liquid, and with reduced corrosion.

While circular sections for conduits and cones are preferred as shown,other polygonal or oval sections can be used. Preferably the samesections are used on all concentric parts to maintain uniformity of flowabout the longitudinal axis of conduit section 22.

A second embodiment of the present invention is shown in FIGS. 5 and 6.A venturi is provided within conduit section 22 by means of truncated,conical member 60 fixed to its walls and having a narrowed throatopening 62. Concentric conical sheet metal members 64 and 66 are mountedbetween the outer wall of member 60 and the inner walls of section 22and define between them a tapering circumferential passageway 67 whichterminates in an annular orifice 68 surrounding throat opening 62.Secondary gas stream A₂ is fed through duct 70 tangentially intopassageway 67, around member 60, and outwardly through orifice 68 whereit is introduced cocurrently into the accelerated effluent gas streamS₂. Relative velocity is provided by feeding stream A₂ at a different,preferably lower, velocity than stream S₂. Both streams are turbulentwith good distributed mixing and efficient impact capture ofcontaminating particulates is obtained. Duct 70 is provided with aflange 71 by which it is joined to the vertical portion of conduit 18 ofFIG. 1 and comprises the outlet portion of that conduit within theconduit 12.

From FIGS. 2-4 and FIGS. 5-6, it will be noted that while the flow iscountercurrent and cocurrent, respectively, there are also radialcomponents of flow as the streams S₂ and A₂ merge. As used herein, theterms countercurrent and cocurrent include such flows where there is asubstantial vector component of motion along or against the direction offlow of the primary effluent stream S₂ parallel to the axis of conduit12. Cocurrent and countercurrent injection of the dispersed targetparticles distributed interiorly of the walls of the conduit arepreferred, with countercurrent flow being most preferred since itprovides the greatest effective relative velocity.

A third embodiment is shown in FIGS. 7 and 8 in which duct 82 terminateswithin conduit section 22 with four nozzles 86 spaced even about thelongitudinal axis of section 22. Nozzles 86 are angled away from thataxis toward the conduit walls and divides the secondary gas stream A₂into four substreams. Duct 82 has flange 84 for joining to verticalconduit 18 and constitutes the upper outlet end thereof within conduit12. Secondary stream A₂ is fed into effluent stream S₂ eithercocurrently or countercurrently, but is preferred countercurrent asshown. Conduit section 22 is provided with access door 80 and the end ofduct 82 is supported within section 22 by means of T-shaped support 88.Since the nozzles 86 and their associated structure significantly reducethe flow area within conduit section 22, gas stream S₂ will beaccelerated as the stream A₂ is introduced therein.

While countercurrent or cocurrent flows are preferred, impact capturecan also be obtained by radial injection from a plurality ofcircumferential positions as shown in the fourth embodiment of FIG. 9,although with reduced efficiency. As shown in FIG. 9, the secondary gasstream A₂ with dispersed target particulates P is fed through conduit 18into a circumferential manifold 92 surrounding the wall of conduitsection 22. Manifold 92 communicates with a plurality of openings 94 inthe wall of section 22 which are preferably uniformly spacedtherearound. Relatively high velocity for stream A₂ and a relativelyeven distribution of flow through openings 94 is required.

Referring again to FIG. 1, the gas streams S₂ and A₂ flow past flange 26as combined stream S₃ and into the cleaning apparatus 14. The effluentgas S₁ has been cooled by admixture with the ambient gas stream A₂ andby any quench liquid applied in chamber 10. If further cooling isdesired, an additional gas stream A₃, controlled by a conventionaldamper or the like (not shown), may be admitted through the duct 100prior to entry into the apparatus 14.

While good results have been obtained without applying electrostaticcharge to the particles P, for some applications the efficiency ofremoval can be increased by providing a charge of either polarity. Thiscan be accomplished, for example by passing the gas stream A₂ withentrained target particulates between charge electrodes 102 and 104within conduit 18. Alternatively, the target particulates can betriboelectrically charged by passing them in contact with a suitabletriboelectric material such as glass or the like located as a lining oras one or more collars within conduit 18. A venturi collar of suchmaterial can be used for increasing friction. The contaminatingparticulates in stream S₁ need not be charged, but can be similarlycharged with the opposite polarity if desired.

EXAMPLE 1

The present invention has been tested in a pilot installation using aslip stream of effluent gas from a glass melting furnace used for themanufacture of glass fibers, employing the apparatus of FIGS. 1 and 5.Ninety percent of the contaminating particulates were less than 1 micronin size and the particulates were estimated to have an average particlesize of about 1/2 micron. These particulates were composed typically ofthe following salts and oxides: sodium fluoride, calcium fluoride,calcium oxides, silica, sodium sulfate and boron oxides. The gas alsocontained acid gas components, particularly oxides of sulfur andhydrogen fluoride as indicated in Table I below. Between about 10 and 35pounds per hour of nepheline syenite having an average particle sizebetween about 10 and 20 microns was metered into the secondary airstream A₂. The incoming gas stream S₁ was quenched in chamber 10 with aspray at the rate of 2.5 gallons per minute with a 2.5% by weight slurryof calcium hydroxide in water. The results of this test are given belowin Table I wherein ACFM means actual cubic feet per minute, PPM meansparts per million, and gr./SCF means grains per standard cubic foot.

                  TABLE I                                                         ______________________________________                                                S.sub.1                                                                              S.sub.2                                                                              A.sub.2  A.sub.3                                                                              S.sub.4                                 ______________________________________                                        GAS VOL.                                                                      ACFM:     7000     5600   1000   1400   7000                                  TEMPER-                                                                       ATURE, °F.:                                                            Dry Bulb: 700      235    Ambient                                                                              Ambient                                                                              170                                   Wet Bulb: 95       134    Ambient                                                                              Ambient                                                                              117                                   Velocity, fps:                                                                           --      50     50-80*  --     --                                   SO.sub.x, ppm:                                                                          100-200                       20-30                                 F.sup.-, ppm:                                                                           90                            1                                     B.sup.-, gr./SCF                                                                        0.25                          0.003                                 Other                                                                         Particulates                                                                  gr./SCF:  0.2                           0.01                                  ______________________________________                                         *at injection into S.sub.2                                               

Capture of the fine contaminating particulates in the effluent gasstream by the target particulates injected with the stream A₂ wasverified by gas elutriation tests. A sample of the separated mixture ofparticulates shaken from the bags in the baghouse was placed in acolumn. A stream of air at various velocities was passed upwardlythrough the particulate sample. Particulates entrained with the air wereseparated, tested, and compared with like tests from the originalmaterial. The tests were substantially the same indicating that the fineparticulates were bound to the heavier target particulates.

EXAMPLE 2

A typical flue gas from a municipal incinerator containing a mixture ofsolid particulates of oxides and salts and treated according to themethod and apparatus of FIGS. 2-4 will give results substantially asfollows, when quenched with 60 gallons per minute of water in chamber 10and supplied with 100 to 150 pounds per hour of target particulates of3-15 micron nepheline syenite in stream A₂ ;

                  TABLE II                                                        ______________________________________                                                     S.sub.1                                                                             S.sub.2  A.sub.2  S.sub.4                                  ______________________________________                                        Gas Vol., ACFM (000's)                                                                       250     150       5     156                                    Temperature, °F.                                                       Dry Bulb:      1600    400      80     380                                    Wet Bulb:      100     164      60     162                                    Velocity, fps, about:  50-80*   50-80*                                        Particulates, gr./SCF:                                                                       2.0                     0.03                                   HCl, PPM:      200                     5                                      HF, PPM:       10                      3                                      ______________________________________                                    

It should be understood that the foregoing description and examples aregiven for the purpose of illustration and that the invention includesall modifications and equivalents within the scope of the appendedclaims.

We claim:
 1. In a method of treating a primary gas stream flowing in aconduit to facilitate the removal therefrom of submicron contaminatingparticulates, wherein a secondary gas stream with target particulatesdispersed therein is introduced into the primary gas stream to promoteinertial impaction between said contaminating and target particulates,the improvement comprising:introducing said secondary gas streamcountercurrently to said primary gas stream through an annular orificecircumscribing an area within said conduit; the average particle size ofthe target particulates being between about 3-50 microns; and deflectingsaid primary gas stream radially outward with reference to thelongitudinal axis of the conduit and away from said area and across saidorifice into an annular flow path, the flow path of the primary streambeing transverse to the countercurrent flow path of the secondary gasstream at said orifice, while simultaneously accelerating said primarygas stream to achieve a relative velocity between said primary andsecondary gas streams at said orifice of about 20-200 feet per secondsuch that the target particulates penetrate the entire primary stream,the primary and secondary streams forming a combined gas stream.
 2. Themethod of claim 1 wherein the relative velocity is greater than about 50feet per second.
 3. The method of claims 1 or 2 comprising the furtherstep of passing the combined gas stream with suspended captured andtarget particulates through means for separating the particulates fromthe gas.
 4. The method of claim 3 wherein the target particulates arenepheline syenite or phonolite.
 5. The method of claim 1 wherein duringintroduction of the secondary gas stream into the primary gas stream,both gas streams are deflected outwardly away from their respective flowaxes towards the conduit wall.